This invention relates to protection systems for light responsive systems, and more particularly to a means for controlling the power emitted by a laser in order to insure eye safety.
The prior art has various shut-off protection systems for light responsive systems. One example may be found in U.S. Pat. No. 4,876,444 to B. F. Field which discloses a photodetector mounted on the laser transmission unit of a light guided vehicle. When a certain type light strikes the photodetector, the photodetector emits an electrical signal. This signal is used to override the laser control causing the vehicle to shut down. Another example may be found in U.S. Pat. No. 4,960,988 to R. A. Simms which discloses a safety shut-off protection system for light responsive systems. The light responsive system includes several, series-connected, normally-closed, light responsive photoelectric Darlington safety switches. These safety switches are located to receive light along with the light responsive systems. Light exceeding a threshold intensity impinging on any one of the Darlington safety switches causes such switches to deactivate the light sensitive system. Although effective for the specific purposes for which the patented devices are specifically disclosed, the above-named patented devices have serious limitations when applied to lasers where eye-safety is the goal.
It is possible in the above patented devices for obstructions (such as dirt, vignetting, etc.) to keep scattered light from striking the protection photodetectors yet not blocking the laser or light-sensitive element itself. The location of photodetectors, as in Simms, could be very dangerous for a laser safety system, because fingers or dirt could block the photodetectors yet allow the laser to strike an object (such as an eye) undetected.
Highly directional scattering or reflections can go undetected by the above prior art systems. If a laser beam strikes a plane mirror at normal incidence, the beam is directed back onto itself. This retroreflection would go undetected by the prior art devices.